Spectroscopy of Planetary Nebulae in Sextans A and Sextans B

Abstract

Abstract. Sextans A and Sextans B are two dIr galaxies situated in the outskirts of the Local Group (LG), in which both PNe and H II regions have been detected (Jacoby & Lesser [5], Magrini et al. [6], [7]). We present spectroscopic observations of PNe and HII regions in these two galaxies (obtained with the VLT) Preliminary results about PNe physico-chemical properties are presented. The galaxies: Sextans A and Sextans BSextans A and Sextans B are both dwarf Irregular galaxies (Ir V and Ir IV-V morphological types, respectively, cf. van den Bergh [12], hereafter vdB00) with approximately the same V luminosity, belonging to the outer fringes of the Local Group. Their distances from the barycenter of the LG are quite similar (1.60 and 1.72 Mpc, see vdB00) and their separation on the sky is relatively small (∼ 10 degree), which corresponds to about 280 kpc. Moreover their velocity difference is only 23 ±6 km s −1 . These results suggest the hypothesis of a common formation of these two galaxies, probably together with NGC 3109 and the Antlia galaxy. Considering the mean distance of the four galaxies from the barycenter of the LG, 1.7 Mpc, this sub-group is located beyond the zero velocity surface of the LG (cf. vdB00) and thus it can be considered the nearest external group of galaxies. As said by Mateo ([9]) "No two Local Group dwarfs have the same star-formation history". This is true also when comparing very similar galaxies. In fact, in spite of a probable common formation, Sextans A and Sextans B show a different star formation history as indicated by the different amounts of stars in the various evolutionary phases, and also reflected in the different number of PNe and HII regions observed. Sextans A shows a large amount of old stellar population, whereas it has a modest intermediate (4-10 Gyrs ago) and recent star formation (1-4 Gyrs ago). On the other hand, Sextans B exhibits a very strong recent star formation (1-4 Gyrs ago), together with a very old population (cf. Mateo's ([9] review about the star formation history of the LG dwarfs).In this context, PNe represent an useful tool to trace the chemical enrichment history of galaxies, moreover when compared with chemical abundance derived